Valves have long been used to control the flow of gases and/or fluids through conduits. In particular, EGR valves have been used to control the amount of recirculated exhaust gas in an automotive engine to thereby reduce the level of nitric oxides Nox) in the exhaust gas. A relatively fast EGR valve response is necessary to maintain low emission levels during transient operation of the engine. EGR valves and valves in general frequently exhibit severe hysteresis impeding a rapid valve response. The hysteresis in a valve may be caused by (i) the paramagnetic behavior of an electrically actuated valve, (ii) dead travel time for a pneumatically actuated valve, and (iii) soot deposits impeding the motion of a closure member within the valve. Regardless of the specific cause of the hysteresis, the hysteresis includes two types of friction: static friction and coulombic friction.
The static friction force (hereinafter referred to as static friction) acts to impede the initial motion of a valve closure member within the valve. Referring to FIG. 1, a diagram illustrating the static friction is provided. The static friction is at a maximum value when the valve closure member is not moving. Once the valve closure member begins moving in either a first direction or a second direction, the static friction is no longer present.
The coulombic friction force (hereinafter referred to as coulombic friction) acts to impede the motion of a valve closure member whenever the closure member is moving.
Referring to FIG. 2, a diagram illustrating coulombic friction is provided. The coulombic friction has a constant amplitude whose sign is dependent on the direction of the velocity of the closure member.
One known throttle valve control system is utilized to control a rotatable throttle valve. The control system includes a control means that increases the effort supplied to the throttle valve when the valve closure member approaches a closed position. A first problem associated with the known control system is that the system only compensates for static friction and friction when the closure member approaches a closed position. Accordingly, when the closure member is at an operational position that is not near the closed position, the control system does not compensate for the static friction. A second problem with the known control system is that the control system does not compensate for coulombic friction. As a result, when the closure member is moving, the response time of the valve may be increased due to the coulombic friction.
There is thus a need for a control system and a method for controlling a valve closure member that minimizes or reduces one of more of the above-mentioned deficiencies.